In order to prevent air pollution and the like, the engine and body of an automobile are provided with various devices for treating harmful emission components. These known devices include, for example, a blow-by gas recirculating device for guiding a blow-by gas, which consists mainly of an unburned fuel component (HC) leaking from a combustion chamber of the engine into a crank case, to an intake pipe, and a fuel evaporative emission suppressing device for guiding a fuel evaporative gas, composed mainly of HC produced in a fuel tank, into the intake pipe.
The fuel evaporative emission suppressing device comprises a canister, loaded with activated charcoal which adsorbs the fuel evaporative gas, various pipes, etc. The canister is provided with an inlet port communicating with the fuel tank, an outlet port communicating with the intake pipe, and a vent port which opens to the atmosphere. In the fuel evaporative emission suppressing device of this canister-storage type, the fuel evaporative gas in the fuel tank is introduced into the canister and adsorbed by the activated charcoal. The atmospheric air (purge air) is introduced into the canister through the vent port by applying a negative pressure in the intake pipe to the outlet port. The fuel evaporative gas adsorbed by the activated charcoal is separated therefrom by means of the purge air, and introduced into the intake pipe along with the purge air. The fuel evaporative gas, thus delivered into the intake pipe, is burned in the combustion chamber of the engine together with an air-fuel mixture, whereby it is prevented from being discharged into the atmosphere.
If the purge air containing the fuel evaporative gas is introduced carelessly into the intake pipe, however, the air-fuel ratio of an air-fuel mixture deviates from its appropriate range, so that the rotational speed and output torque of the engine fluctuate greatly. Accordingly, the comfortableness to drive or drivability of the vehicle worsens. This unfavorable phenomenon is particularly remarkable in the case where the purge air is introduced while the engine is running in an idling speed area in which the quantity of intake air is small.
To avoid this, a purge control valve, for use as purge regulating means for controlling the rate of purge air introduction, is provided in a purge passage which connects the canister and the intake pipe. The purge control valve is opened to allow the purge air to be introduced into the engine only when the engine is operating in a predetermined operation area. In general, purge control valves may be classified into two types, mechanical ones which operate in response to negative intake pressure and electrical ones which are controlled in on-off operation by means of an electronic control unit (ECU) in accordance with pieces of operation information, such as the throttle opening, intake air flow rate, etc. Although the mechanical valves, being low-priced, are widely used, the electrical or solenoid-operated valves are superior in performance, since the introduction and shut-off of the purge air can be controlled more accurately and freely by the electrical ones.
In the fuel evaporative emission suppressing device furnished with such a solenoid-operated purge control valve, however, snapping of wires which connect the ECU and the purge control valve, connector contact failure, etc. may occur, or a valve plug in the control valve may possibly be fixed in a closed state from some cause. In such a case, the purge air cannot be introduced into the intake pipe, so that the canister is overloaded with the fuel evaporative gas. Inevitably, therefore, the fuel evaporative gas additionally supplied from the fuel tank is discharged into the atmosphere without being adsorbed by the activated charcoal.
However, the discharge of the fuel evaporative gas into the atmosphere constitutes no hindrance to the engine operation. Thus, a driver is not aware of this fault as the fuel evaporative gas continues to be discharged into the atmosphere.
A method of making the fault diagnosis of purge control has been proposed in Unexamined Japanese Patent Publication No. H3-286163, etc. With this proposed method, the purge control valve is driven to open during the idling operation of an engine equipped with an idle speed controller (the ISC), and fault diagnosis is made on the basis of the operation state of ISC. When the purge control valve is driven to open during idling operation, purge air is introduced if the purge control valve is normal, whereby the ISC is operated to prevent an increase in engine rotational speed due to the introduction of purge air. On the other hand, if the purge control valve is faulty, purge air is not introduced, so that the ISC is not operated. If the ISC is not operated when the purge control valve is driven to open, the purge control valve is judged to be faulty.
However, the proposed method has a disadvantage in that it can cause erroneous diagnosis. That is to say, in an order to prevent hunting in ISC operation, which occurs when the ISC is operated so as to compensate even a small change in engine rotational speed caused by the variations in fuel condition between engine cylinders or caused by the light load applied to the engine, the ISC is generally operated only when the engine speed deviates from a predetermined range including the target of idling speed during the idling operation. During idling operation, if a running range is selected in an automatic transmission or if a cooler compressor is operated, a relatively heavy load is applied to the engine. At this time, a large quantity of intake air necessary for keeping the engine speed within the predetermined range is supplied. Accordingly, even when purge air is introduced, the increase rate of the quantity of intake air is low, and the rise in engine speed such that the engine speed deviates from the predetermined range does not occur, so that the ISC does not operate. That is to say, in the case where a relatively heavy load is applied to the engine, even if the purge control valve is normal and the purge air for fault diagnosis is introduced normally, the purge control valve is sometimes erroneously judged to be faulty.